A. Introduction
A wide variety of processes have been used in the synthesis of magnetic particles or colloids, which have often been used as MR contrast agents. Klaveness, U.S. Pat. No. 4,863,715; Owen, U.S. Pat. No. 4,795,698; Widder, U.S. Pat. No. 4,849,210; Hasegawa, U.S. Pat. No. 4,101,435; Groman, U.S. Pat. No. 4,827,945; Groman, U.S. Pat. No. 4,770,183; Menz, E. T., International Application, published under the Patent Cooperation Treaty, International Publication No. WO 90/01295, filed Aug. 3, 1989, published Feb. 22, 1990; and Lewis, International Application, published under the Patent Cooperation Treaty, International Publication No. WO 90/01899. These syntheses produce colloids that are heterogeneous and are rapidly removed from blood (or cleared) by phagocytic cells of the reticuloendothelial system (RES). These two features are undesirable in some important respects.
B. Problems with Heterogeneous Magnetic Colloids
The magnetic colloids and particles described previously are either too large or in some cases consist of too many differently sized particles, i.e. exhibit heterogeneity, to be sterilized by passage through a 220 nm filter. Filtration through a 220 nm filter meets the current legal requirement for sterility of parenteral pharmaceutical products. The inability to filter sterilize can result when a very small proportion of the colloidal material is large enough to be trapped on the filter, thereby blocking further filtration.
Heterogeneous colloids, or colloids composed of many subpopulations with a wide range of properties, can be difficult to characterize. Heterogeneity can obscure the detection of small atypical populations, such atypical populations being present either at the time of manufacture or developing with storage. With homogeneous colloids, a single population with a narrow range of properties comprises the colloid, and the detection of small, atypical subpopulations is facilitated. In the manufacture of superparamagnetic iron oxide colloids for injection into humans, atypical subpopulations of colloidal materials are considered to be potential sources of toxicity. Consequently, the ability to assure the absence of such subpopulations improves the quality of the colloid that can be produced.
Homogeneous colloids have previously been prepared by fractionation of heterogeneous colloids. For example, the heterogeneous colloid AMI-25 has been fractionated by Sepharose 4B column chromatography to yield four colloids of differing size, see Table I of Lewis, et al., International Application, published under the Patent Cooperation Treaty, International Publication No. WO 90/01899. AMI-25 is prepared according to Example 7.10 of U.S. Pat. No. 4,827,945. These experiments demonstrated that the size of superparamagnetic iron oxide colloids can play a role in controlling pharmacokinetics. As the size of the colloidal particle dropped below about 100 nm, clearance decreased, i.e. plasma half-life increased.
Dextran magnetite, prepared as described (Hasegawa, U.S. Pat. No. 4,101,435), is another example of a heterogeneous colloid. Dextran magnetite contains particles with diameters from 20 to 520 nm, median 160 nm. Like AMI-25, dextran magnetite has been subjected to size fractionation, with smaller fractions exhibiting longer blood half-lives. The overall preparation has a very short plasma half life of 3.5 minutes, the smallest fraction having a half-life of 19 minutes and the largest 0.9 minutes. Iannone, N. A. and Magin, R. L. et al., "Blood Clearance of Dextran Magnetite Particles Determined by a Non-Invasive In Vivo ESR Method" submitted to J. Mag. Res. Med.; Magin, R. L., Bacic G. et al. "Dextran Magnetite as a Liver Contrast Agent," (1991) Mag. Res. Med. 20 pp. 1-16.
In a synthesis described by Molday (U.S. Pat. No. 4,452,773) a heterogeneous colloid is produced, with the large aggregates being removed by centrifugation and discarded (column 8, line 40). The preparation of homogenous colloids from heterogeneous colloids is wasteful because unwanted colloidal material is discarded. In addition, the fractionation step itself is time consuming and expensive.
A second problem with the magnetic colloids and particles cited above is that after injection they exhibit rapid clearance by the tissues of the reticuloendothelial system (RES). Rapid clearance (short plasma half-life) limits the time the injected material is present in the blood pool of an animal. In some MR imaging applications, for example in the delineation of the capillary blood volumes of different parts of the brain, image contrast is enhanced by the presence of superparamagnetic iron oxide in the blood. Bradley et al. (1989) Stroke 20 pp. 1032-1036. A long plasma half-life lowers the dose of colloid needed and prolongs the time when a contrast enhanced MR image can be obtained.
The rapid clearance of prior art magnetic colloids or particles from the blood results from their uptake by the liver and spleen, the two major tissues of the RES. With rapidly cleared particles, typically about 90% of the injected material is extracted from blood by those two organs. Therefore rapid blood clearance limits the amount of colloid that can be targeted or delivered to cells other than phagocytes and tissues other than the liver and spleen.
There have been reports that surface chemistry can affect the fate of colloids after injection, for example results with a heterogenous colloid made with arabinogalactan. Josephson et al., (1990) Mag. Res. Imag. 8 pp. 637-646; and Example 6.10.1 of Menz, E. T. International Application, published under the Patent Cooperation Treaty, International Publication No. WO 90/01295, filed Aug. 3, 1989, published Feb. 22, 1990. Similarly, there have been reports that attaching antibodies to iron oxide particles can achieve tissue specific uptake of the iron. Renshaw et al. (1986) Mag. Res. Imag. 4 pp. 351-357; and Cerdan et al., (1989) Mag. Res. Med. 12 pp. 151-163. However, such preparations are often heterogenous, cannot be filter sterilized, or show very substantial uptake by the liver and spleen. Uptake by the liver and spleen limits the amount of iron that can be targeted to other tissues.